Being an American of "a certain age", any time I hear the word "inquiring" I immediately think of the National Enquirer ads that were plastered all over US radio and TV in the 80's, with the catchphrase "inquiring minds want to know". The magazine in question was largely sold as an impulse item at supermarket checkout lines, and was chock full of attention-grabbing ridiculous stories. Sort of a pre-web version of clickbait.
I believe the ads were an attempt to rehabilitate the image of their brand, which failed spectacularly. It became a huge joke, particularly on the late night TV circuit. Today, we'd call it a meme. The phrase became a roundabout way of calling someone stupid. But it was such a huge meme, the Enquirer kept with it for publicity's sake (if anybody in the world understands the sales value of a meme, believe me it is the folks running tabloids). They even trademarked the phrase in 1987.
So if you say "inquiring minds" to a US person, they will likely think you are talking about stupid or shallow people. The kind who might purchase and actually believe and get excited about articles in a cheap supermarket tabloid. That's almost certainly why you got "corrected".
For that reason, I don't think the word "inquiring" has been used for its original meaning in the USA much in the last 30 years. Perhaps that will start to change, now that we have an increasing amount of younger people who didn't have that phrase beaten into their heads.
"Inquisitive" of course didn't suffer from use in such an ad, and I believe mostly has positive connotations in the US.
The answer to a "proper" expression for underground floors is that there is no standardized nomenclature, rather, there are multiple different designators depending on the place in question and the country: https://en.wikipedia.org/wiki/Storey#Subterranean_floors
The numbering of levels below ground is also quite varied, even within the same country. In English-speaking countries, the first level below ground may be labelled B for "Basement", LL for "Lower Level" or "Lower Lobby", C for "Cellar" or, in the case of the Andy Warhol Museum in Pittsburgh, U for "Underground". In British buildings, LG for "Lower Ground" is commonly encountered (or lower ground floor).
If there is more than one basement, the next level down may be marked SB for "Sub-Basement". The lower levels may also be numbered B1, B2, B3, B4, B5, B6, B7, B8, B9, B10, etc.. Negative numbers are sometimes used: −1 for the first level below ground, −2 for the second one, −3, −4, −5, −6, −7, −8, −9, −10, etc. Lettered levels are also sometimes used: A, B, C, D, E, F, G, H, I, J, etc.
In Hawaiian, these levels are numbered I1, I2, I3, and so on. The "I" stands for "Ilelo", meaning basement.
Of course, that's not to say that people won't understand you if you use first floor underground, but I think the reason this isn't widely used is because it's just a mouthful. Five syllables, versus the three syllables of basement 1, or first basement.
First floor underground has been used, though, and can be found in various engineering books, particularly in the English translation of Chinese manuals:
Code for Fire Protection Design of Buildings
"If they are arranged underground or semi-underground, they should be arranged on the first floor underground but shall not be arranged on the third floor underground or below."
The 2nd International Symposium on Rail Transit Comprehensive Development ...
"Commercial space is located on the podium and the first floor underground, while the commercial apartment is located in the high-rise building on the north side."
Advances in Civil Engineering and Building Materials
"The first floor underground chooses the zoning control combined with the light tube during the day and close corresponding area lights and it is open the evening. The second floor underground is also lighting for garage."
Rainbow Town
"These two were the ones who said they wanted to stay on the first floor underground"
Civil Engineering in Japan, Volumes 6-8
"View of the illumination towers from the vicinity of the taxi station, the first floor underground."
CCDI Architecture: Interaction with a Complex Context
" The commercial building comprises the main department stores and retail business, with the supermarket and retail space on the first floor underground."
Re/collecting Early Asian America: Essays in Cultural History
"The first floor underground was the home of the "number two" employees. In the Chinese social code actors belonged in the lower strata."
...and more can be found in Google Books
Ngrams shows that basement floor is the outright winner compared to underground floor/floor underground/basement level/floor basement/lower ground floor/lower lobby/lower level floor/lower floor level/sub-basement/subbasement/sub basement. I didn't include lower level because the results would be corrupted by the common definition of level.
Best Answer
This expression, whether in its noun form have a beef, or its verb form, to beef, may come down to us from Cockney rhyming slang.
Beef rhymes with Thief
Beeves (archaic) rhymes with Thieves
Imagine a bustling market day. In the narrow streets, a neighborhood pickpocket weaves through the crowd, pursued by a stranger shouting, "Stop! Thief!" Imagine that none too few persons in the market know the thief, or consider themselves bound to him by common interest. To muffle the alarm, the bemused sympathizers call out "Hot Beef!" and amidst the confusion the rascal slips away.
Too fanciful for you? Well, then, if you will, accept only that Cockney rhyming slang is a real phenomenon, without delving into the reasons for its adoption.
From Historically Speaking:
Here are four related entries from the Dictionary of the Vulgar Tongue (1811):